1. Field of the Invention
Generally, the present disclosure relates to the manufacture of semiconductor devices, and, more specifically, to various methods of forming a masking layer for use in patterning underlying structures.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPUs, storage devices, ASICs (application specific integrated circuits) and the like, requires a large number of circuit elements, such as transistors, capacitors, resistors, etc., to be formed on a given chip area according to a specified circuit layout. During the fabrication of complex integrated circuits using, for instance, MOS (Metal-Oxide-Semiconductor) technology, millions of transistors, e.g., N-channel transistors (NFETs) and/or P-channel transistors (PFETs), are formed on a substrate including a crystalline semiconductor layer. A field effect transistor, irrespective of whether an NFET transistor or a PFET transistor is considered, typically includes doped source and drain regions that are formed in a semiconducting substrate and separated by a channel region. A gate insulation layer is positioned above the channel region and a conductive gate electrode is positioned above the gate insulation layer. By applying an appropriate voltage to the gate electrode, the channel region becomes conductive and current is allowed to flow from the source region to the drain region.
To improve the operating speed of field effect transistors (FETs), and to increase the density of FETs on an integrated circuit device, device designers have greatly reduced the physical size of FETs over the past decades. More specifically, the channel length of FETs has been significantly decreased, which has resulted in improving the switching speed of FETs and the overall functionality of the circuit. Further scaling (reduction in size) of the channel length of transistors is anticipated in the future. While this ongoing and continuing decrease in the channel length of transistor devices has improved the operating speed of the transistors and integrated circuits that are formed using such transistors, there are certain problems that arise with the ongoing shrinkage of feature sizes that may at least partially offset the advantages obtained by such feature size reduction. For example, as the channel length is decreased, the pitch between adjacent transistors likewise decreases, thereby increasing the density of transistors per unit area. This scaling also limits the size of the conductive contact elements and structures, which has the effect of increasing their electrical resistance. In general, the reduction in feature size and increased packing density makes everything more crowded on modern integrated circuit devices.
Typically, due to the large number of circuit elements and the required complex layout of modern integrated circuits, the electrical connections of the individual circuit elements cannot be established within the same level on which the circuit elements, such as transistors, are manufactured. Rather, modern integrated circuit products have multiple so-called metallization layer levels that, collectively, contain the “wiring” pattern for the product, i.e., the conductive structures that provide electrical connection to the transistors and the circuits, such as conductive vias and conductive metal lines. In general, the conductive metal lines are used to provide intra-level (same level) electrical connections, while interlevel (between levels) connections or vertical connections are referred to as vias. In short, the vertically oriented conductive via structures provide the electrical connection between the various stacked metallization layers. Accordingly, the electrical resistance of such conductive structures, e.g., lines and vias, becomes a significant issue in the overall design of an integrated circuit product, since the cross-sectional area of these elements is correspondingly decreased, which may have a significant influence on the effective electrical resistance and overall performance of the final product or circuit.
Improving the functionality and performance capability of various metallization systems has also become an important aspect of designing modern semiconductor devices. One example of such improvements is reflected in the increased use of copper metallization systems in integrated circuit devices and the use of so-called “low-k” dielectric materials (materials having a dielectric constant less than about 3) in such devices. Copper metallization systems exhibit improved electrical conductivity as compared to, for example, prior metallization systems that used aluminum for the conductive lines and vias. The use of low-k dielectric materials tends to improve the signal-to-noise ratio (S/N ratio) by reducing cross-talk as compared to other dielectric materials with higher dielectric constants. However, the use of such low-k dielectric materials can be problematic as they tend to be less resistant to metal migration as compared to some other dielectric materials.
Copper is a material that is difficult to directly etch using traditional masking and etching techniques. Thus, conductive copper structures, e.g., conductive lines or vias, in modern integrated circuit devices are typically formed using known single or dual damascene techniques. In general, the damascene technique involves (1) forming a trench/via in a layer of insulating material, (2) depositing one or more relatively thin barrier or liner layers (e.g., TiN, Ta, TaN), (3) forming copper material across the substrate and in the trench/via, and (4) performing a chemical mechanical polishing process to remove the excess portions of the copper material and the barrier layer(s) positioned outside of the trench/via to define the final conductive copper structure. The copper material is typically formed by performing an electrochemical copper deposition process after a thin conductive copper seed layer is deposited by physical vapor deposition on the barrier layer.
Photolithography is one of the basic processes used in manufacturing integrated circuit products. At a very high level, photolithography involves: (1) forming a layer of light or radiation-sensitive material, such as a photoresist material, above a layer of material or a substrate; (2) selectively exposing the radiation-sensitive material to a light generated by a light source (such as a DUV or EUV source) to transfer a pattern defined by a mask or reticle (interchangeable terms as used herein) to the radiation-sensitive material; and (3) developing the exposed layer of radiation-sensitive material to define a patterned mask layer. Various process operations, such as etching or ion implantation processes, may then be performed on the underlying layer of material or substrate through the patterned mask layer.
Of course, the ultimate goal in integrated circuit fabrication is to faithfully reproduce the final circuit layout (design) on the integrated circuit product. Historically, the pitches employed in integrated circuit products were large enough such that a desired pattern could be formed using a single patterned photoresist masking layer. However, in recent years, device dimensions and pitches have been reduced in size to the point where existing photolithography tools, e.g., 193 nm wavelength photolithography tools, cannot form a single patterned mask layer with all of the features of the overall target pattern. That is, existing 193 mm wavelength photolithography tools and techniques are limited to printing patterns having a pattern pitch above 70 nm using a single layer of photoresist. Accordingly, device designers have resorted to techniques that involve performing multiple exposures to define a single target pattern in a layer of material. One such technique is generally referred to as double patterning or double patterning technology (DPT). In general, double patterning is an exposure method that involves splitting (i.e., dividing or separating) a dense overall target circuit pattern into two separate, less-dense patterns. The simplified, less-dense patterns are then printed separately utilizing two separate masks (where one of the masks is utilized to image one of the less-dense patterns, and the other mask is utilized to image the other less-dense pattern). Further, in some cases, the second pattern is printed in between the lines of the first pattern such that the imaged wafer has, for example, a feature pitch which is half that found on either of the two less-dense masks. This technique effectively enables the printing of even smaller features than would otherwise be possible using a single mask using existing photolithography tools. There are several double patterning techniques employed by semiconductor manufacturers.
While such double patterning techniques can enable the printing of features with pitches less than can be formed using a single layer of patterned photoresist material, such double patterning processes are time-consuming and require a great deal of precision in terms of overlay accuracy. So-called sidewall image transfer techniques can also be employed to form patterns having reduced pitches, but such sidewall image transfer techniques are time-consuming and expensive.
The present disclosure is directed to various methods of forming a masking layer for use in patterning underlying structures that may solve or at least reduce some of the problems identified above.